The present invention relates to a nuclear reactor vibration surveillance system for structural integrity monitoring of a nuclear reactor internal component and its method.
As a technique for checking the structural health of a nuclear reactor internal component, there is known a method that measures the vibration amplitude or vibration frequency of the reactor internal component and evaluates the measurement values or trend thereof. In order to measure the vibration of the reactor internal component, a method is generally adopted in which a vibration sensor is installed in the reactor, and a signal from the vibration sensor is sent outside the reactor by means of a signal cable.
However, in this method, it is necessary to lay the signal cable inside and outside the nuclear reactor, so that it takes quite a lot of work to prepare for the measurement.
In order to cope with this problem, an ultrasonic vibration measurement system has been proposed as disclosed in Japanese Patent No. 3,782,559 (the entire content of which is incorporated herein by reference).
In this ultrasonic vibration measurement system, an ultrasonic sensor is placed on the outer surface of a reactor pressure vessel, and an ultrasonic pulse is allowed to propagate in the reactor through the reactor pressure vessel. The ultrasonic pulse propagating in the reactor collides with and reflected by a reactor internal component such as a shroud or jet pump under water. The reflected ultrasonic pulse is retuned once again to the ultrasonic sensor through the reactor pressure vessel.
If the reactor internal component such as a shroud vibrates, the propagation time of the returned reflected ultrasonic pulse is slightly changed due to the vibration. Assuming that the change in the propagation time of the ultrasonic pulse is Δt (sec), the vibration amplitude L (m) of the reactor internal component such as a shroud is calculated by using the following equation (1).
                              Δ          ⁢                                          ⁢          L                =                              C            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            t                    2                                    (        1        )            
“C” is the acoustic velocity (m/sec) of reactor water.
By plotting the ΔL in time series, the vibration waveform of the reactor internal component such as a shroud can be synthesized.
In the conventional nuclear reactor vibration surveillance system and its method, an ultrasonic sensor employed is configured both as a transmitter and receiver of the ultrasonic. Thus, for example, in the case where the reactor internal component such as a jet pump is arranged inclined relative to the reactor pressure vessel, the ultrasonic pulse is obliquely reflected by the jet pump, so that the reflected ultrasonic pulse is not returned to the ultrasonic source position. As a result, the ultrasonic sensor cannot receive the reflected ultrasonic pulse, making it impossible to measure the vibration of the reactor internal component.
The present invention has been made to solve the above problem, and an object thereof is to provide a nuclear reactor vibration surveillance system for structural integrity monitoring of a nuclear reactor internal component and its method capable of measuring the vibration of an in-reactor stricture arranged inclined relative to a reactor pressure vessel.